Method and apparatus for explosive drilling of well bores



June 23, 1970 J. D. BENNETT ET AL 3,515,502

METHOD AND APPARATUS FOR EXPLOSIVE DRILLING OF WELL BORES Filed July 9, 1968 ACC UMULATOR' TO 33 DRILL PIPE INVENTOR JOHN D. BENNETT FORD L. JOHNSON FRED M. MAYES JOHN W. PERET United States Patent 3,516,52 Patented June 23, 1970 3,516,502 METHOD AND APPARATUS FOR EXPLOSIVE DRILLING 0F WELL BORES John D. Bennett, Richardson, Tex., Ford L. Johnson, Fallbrook, Calif., and Fred M. Mayes, Richardson, and John W. Peret, Dallas, Tex., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Filed July 9, 1968, Ser. No. 743,426

Int. Cl. E21b 7/00 U.S. Cl. 175-45 10 Claims ABSTRACT OF THE DISCLOSURE The particular embodiment described herein as illustrative of one form of the invention utilizes, in a drilling operation, a system for introducing alternate slugs of liquid and gas into the Well bore as the drilling fluid. At the same time, explosive capsules are introduced into the drilling fluid, which explosives are arranged to detonate upon impact with the earth formation being drilled.

BACKGROUND OF THE INVENTION This invention relates generally to methods and apparatus for drilling well bores, and more particularly to a system for increasing the drilling rate by introducing explosive charges together with a liquid gas phase drilling fluid into the drilling fluid system.

In order to point out the advances represented by the invention disclosed herein, it will be helpful first to consider briefly, certain aspects of standard drilling practices. In rotary drilling, a drill bit is carried or rotated by a hollow drill stem, through which a heavy drilling fluid is pumped, which is usually termed mud. The mud emerges from the drill stem through the bit to lubricate the cutters on the bit in their cutting action, and to flush away the cuttings which are then brought to the surface by the return column of mud surrounding the drill stem. The mud penetrates exposed interstices, or crevices of the formation, through which the cut is being made. This intrusion of mud which is in the form of a mud cake about the borehole wall, serves to hold formation pressure in check, and also prevents the walls of the bore from caving or breaking doWn. Such mudding off of the bore during drilling operations maintains the well in a dead condition throughout the drilling operation, with the mud cake and circulating pressure of the mud preventing the inflow of formation gas, oil, or other formation fluids. Normally, the Weight or gravity of the mud is maintained at a level such that the hydrostatic head produced by the column of mud in the well bore will be sufficient to maintain a positive differential back pressure over the expected downhole formation pressure.

It has been found by experiment that not only does the mud cake form about the borehole wall, but also at the bottom of the borehole which is being drilled by the drill bit. Normally, as the bit grinds into the earth formation, the mud is continuously 'being applied to the bottom of the hole, so that a positive back pressure is maintained against the formation being drilled at all times. Such a positive pressure on the formation at the bottom of the borehole is a direct deterrent to the removal and lifting of cuttings from the borehole, with the pressure tending to hold the cuttings down rather than permitting their removal to the surface with the mud column. If such mud pressure can be reduced on the formation at the floor of the borehole, the internal formation pressures themselves will then provide a positive expelling force for the portions of the formation being cut by the drill bit. Such an action would substantially increase the drilling rate, and thereby effect economies in the drilling operation.

According to a paper by W. C. Maurer, published in the Journal of Petroleum Technology, December, 1965, studies of rock failures under a bit show that with a low positive differential pressure on the formation, softer rock formations such as limestone and sandstone fail in a brittle manner, and a large column of rock is removed from under the bit teeth. Under high differential pressure, how ever, the same rock fails in a pseudo plastic manner, and has physical characteristics similar to much harder rock formations. Therefore, when drilling under such conditions, a much smaller volume of rock is removed. Also, at high differential pressures, the cratering of the formation by the drill in unconsolidated sand is similar to that in solid rock because of high friction present between the sand grains. Studies of laboratory drilling rates on a quantitative basis in comparison to differential pressures show that at a high differential pressure, 3,000 or 4,000 p.s.i. over balance, even a rather large change in differential pressure produces only a very small change in laboratory drilling rate, or crater volume under the bit tooth. However, as the differential approaches zero, from about 500 p.s.i. overbalance on down to the balance point, the drilling rate increases 3 or 4 fold, and this of course is very significant. Field studies also confirm these relationships which have been found in laboratory research. It has been found that at very high overbalance, there is very little change in drilling rate, with changes in differential pressure until the differential reaches the 500 p.s.i. point and approaches the actual balance point. In this interval, a very drastic increase in drilling rate takes place, with further decreases in differential pressure. In normal drilling operations, when a bit tooth impacts a rock, a Wedge of crushed rock is formed beneath the tooth. As the force on the tooth is increased, the wedge is compressed until a threshold force is exceeded, and a crater is formed. An increase in differential pressure normally increases the threshold force, and likewise increases in mud Weight can effect such increases in threshold force. The threshold contact pressure in granite or basalt may range from 300,000 to 600,000 p.s.i. which exceeds the contact pressure under roller-bit teeth. Under such conditions, it is necessary to use other types of bits such as button or diamond bits.

Recent advancements in pressure detection technology have led to more precise techniques for the detection of formation pressures, so that pressure balances may be more closely controlled. Such detection techniques would also be applicable to the present invention as a means for maintaining proper pressure controls in the Well bore when using gas slugs as a drilling aid.

According to the above referenced paper by W. C. Maurer, under simulated air drilling conditions, increases in depth of hole (overburden pressure) and confining pressure, produced no detectable changes in the threshold force. Good bottom hole cleaning is present in air-drilling because the cuttings can easily be removed from the crater. This, plus the fact that the crater volumes are larger due to the elimination of positive differential pressure in the formation, explains why air drilling is usually much faster than mud drilling.

There are, of course, drawbacks to the use of air as a drilling fluid. While mud-laden fluid or drilling mud can be used to support incompetent formations which have been drilled to control high pressure encountered in water, gas, and oil-bearing zones; air cannot readily serve these purposes. Normally, its use is restricted to those areas where the walls of the hole in the formations penetrated will retain their position without support. In other Words, air and gas are normally suitable for drilling in hard rock without high pressure fluids. Two other vital functions of mud-laden fluid or water are the removal of cuttings and the cleaning and cooling of the bit. In formations suitable for air drilling, these two operations are performed by air in a manner generally superior to mud.

Another technique of drilling which is being developed involves the use of explosives introduced into the mud stream and carried through the drill stern and out the eye of the bit. Upon contact with the formation, the explosive is detonated to assist in the drilling operation Which would normally be continued during this process. The purpose of effecting the explosions is readily understood. If hard rock is being drilled, there is a tendency for the drill to rotate on the surface of the rock with a relatively low rate of rock removal despite heavy pressure on the bit. In other words, the bit pressure does not exceed the threshold pressure of the rock. However, if the bit encounters rock which has been fractured, the cutting edges of the bit will enter openings due to the fractures and tend to split the rock. along such fractures. By this mechanism, comminution is rapidly effected to produce rock fragments of small size removable by drilling fluid circulation. The use of the explosive, when used in the manner described, is not primarily for the purpose of having the explosive actually shape the hole, but rather the objective is to break up the rock to cause the bit to function more effectively.

The objectives of the invention will 'be evident from the foregoing, and may be broadly defined as to provide a new and improved technique for drilling well bores.

SUMMARY OF THE INVENTION With this and other objects in view, the present invention contemplates a method of drilling into earth formations while utilizing a drilling fluid comprised of alternate slugs of a liquid and a gaseous material, while at the same time introducing explosives into the drilling fluid during the drilling operation.

Apparatus is provided for accumulating volumes of the liquid and gas at the surface, with automatic means for introducing the slugs into the drill stem, and in addition, surface equipment is provided for introducing the explosives into the drilling fluid system.

A complete understanding of this invention may be had by reference to the following detailed description, when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a well bore, including a drill stem and bit utilized in a rotary drilling operation, and illustrating the principles of the present invention;

FIG. 2 is a schematic illustration of surface equipment utilized in the system of the present invention;

FIG. 3 is a partial schematic view of the well bore illustrating the drilling operations during a liquid phase of mud circulation about the bit; and

FIG. 4 is an enlarged view of the drill bit of FIG. 3 showing an explosive device projecting from the bit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 of the drawings, a drill stem 12 is shown extending downwardly within a well bore 14. The drill stem 12 is comprised of sections of drill pipe, and includes a bit 16 positioned at its lower end. The bit includes a rotating member having teeth 18 thereon, for removing portions of the formation to accomplish the drilling operation. The drill pipe 12 has a hollow interior bore 20 which communicates with an interior bore 22 within the drill bit. One or more passages 24 communicate the bore 22 in the bit with the exterior of the bit along its lower side. The bores within the drill stem and bit, including the communicating passages 24 are normally provided for the purpose of passing a drilling fluid from the surface through the drill stem and out the lower end of the bit during the drilling operation. Such circulation of fluid provides a means for lubricating the bit, carrying removed portions of the formation 4 to the surface, and providing a hydrostatic back pressure against formation pressures.

Various drilling fluid systems are utilized in order to maintain proper pressure control within the well bore. For example, if high bottom-hole pressures are expected to be encountered, heavy drilling fluids are used which may be two or more times as dense as water. Water also has been used as a drilling fluid in order to maintain a low pressure against the formation, which in turn normally increases the rate of drilling. Lighter than water fluids, such as oil, are also used in drilling operations as well as aerated fluids which tend to further decrease the weight of the drilling fluid. Air and other gases are also used as the fluid under certain conditions. None of the above described systems however, provide the advantages of the present system, which includes, as shown in FIG. 2, a means for introducing alternate slugs of a gas and liquid together with an explosive through the drill stem for purposes to be hereinafter described.

As shown schematically in FIG. 2, the surface equipment includes a mud pump 30, which is connected by an appropriate piping system to a mud accumulator 32. The accumulator discharges through a line 33 into a programmed valve 34. An air pump 36 is connected by piping to an air accumulator 38, which in turn is also communicated by means of line 35 with the programmed valve 34. The valve 34 has a discharge line or pipe 40, which connects with the drill stem by means of conventional drilling fluid system apparatus. The surface equipment of FIG. 2 also includes means for introducing eX- plosive capsules into the drilling fluid system. Such means includes a pipe 37 which connects with the discharge line 40 by means of a Y fitting 39. The pipe 37 contains a straight section having spaced valves 41, 43. The space between valves 41, 43 forms a trap chamber 47.

In the operation of the apparatus thus far described, the mud and air pumps 30 and 36, respectively, are operated continuously to maintain a supply of liquid and gas to the respective accumulators 32 and 38, which in turn feed to the programmed valve 34. The valve is operated to connect alternate flow channels between the pipe 40 and communicating lines with accumulators 32, 38. The programmed valve is operated in a timed sequence so that a desired volume of liquid or gas is alternately supplied to the pipe 40, which in turn communicates with the interior of the drill pipe through a conventional swivel arrangement schematically represented at 42. Drive means (not shown) would of course be supplied, for example, if a rotary drilling operation were used, for rotating the drill pipe and bit.

The trap chamber 47, in the drilling fluid system, is operated as follows: the valve 41 is first opened to permit the insertion of an explosive capsu e within the chamber 47. The outer valve 41 is then closed. When it is desired to inject the capsule into the mud stream, valve 43 is open and the flow through line 40 draws the capsule into the drilling fluid system. It is readily seen that an automatic system for introducing the capsules could be devised and such a system is suggested in US. Pat. No. 3,130,797.

As shown in FIG. 1, alternate slugs of mud 44 and air 46, after being introduced into the drill pipe 12, progress downwardly within the interior bore of the pipe. The air slug is compressed under the influence of the increasing column of weight on the air slug as it progresses downwardly within the pipe. Therefore, it is necessary to introduce a very large volume of air into the drill pipe at the surface in order to provide a suflicient volume of air or gas at the bottom of the well bore for practicing the invention described herein. For example, if a 12 pound mud or drilling fluid were used in the drilling operation for purposes of balancing pressure forces within the formations traversed by the well bore, a gas slug at a position 10,000 ft. below the surface would be subjected to approximately 400 atmospheres. This approximation does not, however, take into effect the lessened weight of the mud column due to other air slugs being entrained therein, or temperature considerations which would increase with the depth of the well, and therefore would tend to increase the volume of gas. For example, 200 to 400 times as much gas would be introduced into the drill pipe at the surface, than would be required at the bottom of the well bore for performing the method according to the present invention.

As the alternate slugs of liquid and gas reach the bottom of a well bore, and provide such alternate environments in which the bit operates, the conditions as suggested in FIGS. 1 and 3 would exist. First referring to FIG. 1, when the air slug reaches the bottom of the drill stem and emerges into the well bore to provide a gas environment about the bit, the mud cake (represented by the darkened area in the well bore) which is normally present on the bottom of the well bore, is removed by the rotating drill bit. This permits the gas to penetrate into the formation below the bit. In addition, the high velocity gas emitting through the passage 24 tends to also help remove the mud cake from the bottom of the bore. Penetration of the gas phase into the formation permits a pressure equalization or positive pressure under the chips or formation fragments to reduce the chip holddown forces normally encountered when drilling in the liquid phase. It is pointed out that the gas phase of the drilling fluid might be carbon dioxide or engine exhaust gas to decrease the danger of an explosion in the mud system, as for example, if ignitable formation gas were encountered during the drilling operation. However, even if the gas phase is combustible, the alternate liquid phases will act as a fire extinguisher in that the liquid phase will not normally support combustion.

As shown diagrammatically by the arrows in FIG. 1, the gas phase penetrates into the formation below the bit to provide such equalizing pressures above and below portions of the formation being removed by the bit. This in itself provides for a higher drilling rate in that it tends to eliminate the holddown pressure, which is normally exerted by a positive differential pressure in the lower end of the well bore. Elimination of the holddown pressure permits the formation fragments to be readily lifted by the circulating drilling fluid for removal to the surface. It is expected that during the gas phase, the pressure around the bit is somewhat higher than during the liquid phase due to the rapid expansion of a gas through the bit passage 24 which causes a surge of pressure greater than that exerted by the drilling fluid column in the well annulus.

Referring next to FIG. 3, it is seen that when the liquid phase of the drilling fluid occupies the lower end of the well bore, a mud cake is again formed on the bottom of the bore. However, the higher pressure gas (represented by the arrows in FIG. 3) which has penetrated the formation, tends to move upwardly through the cake toward the lower pressure environment of the liquid phase to provide a negative differential pressure in the well bore which tends to move the chips and formation fragments upwardly from the bottom of the well bore. Even if no differential pressure were to exist across the mud cake after the liquid phase had entered the lower end of the bore, a mere equalization of pressure across the mud cake would provide for an increased drilling rate in that the formation fragments beneath the bit would not be subjected to the positive holddown pressure which exists with a conventional single-phase drilling fluid system. It is seen that the fluctuation of drilling fluid pressure, which occurs as alternate slugs 'of gas and liquid occupy the lower end of the well bore, will cause a cyclical surging of the formation face at the bottom of the well bore. Such dynamic forces will benefit the removal of formation fragments and increase the rate of drilling.

The improvement which comprises the present invention includes the use of explosive capsules in a drilling operation which utilizes alternate slugs of gas and liquid as the drilling environment. The explosive capsules 45 are introduced into the mud stream by means of the injection mechanism described above. For example, if the capsule is introduced during the liquid phase of drilling fluid, the capsule 45 is carried through the drill stem and out the passage 24 in the bit as shown in FIGS. 1 and 4. As the capsule approaches the passage 24 in the bit, the acceleration of fluids through the passage cause the capsule to align with the passage (FIG. 3). The accelerating fluid also supplies an impact force to the capsule which serves as a means for detonating the capsule as it is impacted against the formation at the bottom of the borehole. The explosive charge in the capsule is preferably in the form of a shaped charge as set forth in detail in US. Pat. No. 3,130,797. Upon detonation, the explosive, particularly in hard rock, will cause a shattering or fracturing effect. This, in turn produces channels within the rock to accommodate the advantageous effects of the alternate slugs of gas and liquid as set forth above. This fragmentation or rock directly aids the drilling operation in that the bit energy is supplemented by explosive energy. In addition, however, the fractures created by the explosives offer flow paths for fluids under pressure to enter the rock, and thereby provide a positive lifting pressure or equalizing pressure to the fragmented rock to oppose the effect of drilling fluid pressure which tends to hold down rock fragments. The liquid phase will also penetrate the fractures created by the explosion to assist in rock breakage by virtue of hydraulic force transmission.

The explosive capsule may be transported through the drill stem with either the liquid or gas phase. If transported in the liquid phase, a greater ta'mping effect is provided by the density of the liquid to provide backup resistance to the capsule explosion thereby giving better utilization or direction to the energy expended.

Upon return of the drilling fluid to the surface through the annular space between the drill pipe and the well bore, the compressed slugs of gas will tend to expand back to the original volume which they occupied upon introduction into the drill pipe. Although there may be considerable intermixing of the liquid and gas phases during the return trip of the drilling fluid to the surface, it is likely that there will still be some slugs of air present in the drilling fluid when it reaches the surfaces, and therefore, certain precautions must be taken to provide control measures in the system which will prevent uncontrolled flow of drilling fluids at the surface. For example, the blowout preventers, which would normally be open during a drilling operation except to provide safety precautions would, in the system disclosed herein, probably be maintained is a closed condition with suitable sealing means 49 provided between the upper closure on the well and the rotating drilling pipe to provide a sealed upper end to the well bore. A flow line is connected to the surface casing to permit the exit of drilling fluids into a fluid recovery system at the surface.

In order to accommodate the return of fluids entrained with gas or having slugs of gas, the recovery system would include some sort of degassing equipment which is well known in the art for degassing drilling mud. In such a system the flow line empties into a separator (not shown) for removing the gaseous portions of the drilling fluid and permitting the liquid and solid portions to escape from the separator. Such escaping materials would then pass over a shale shaker or a similar apparatus for removing the solid particles or formation chips from the drilling fluid. The liquid portions of the fluid then pass into a recovery vessel or pit (not shown) for recirculation in the drilling fluid system.

While a particular embodiment of the present invention has been shown and described, it is apparent that 7 changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is: 1. A method for drilling a well bore into earth formations comprising the steps of: operating a drilling member in the earth formation for removing portions from the earth formation; passing alternate slugs of a liquid and a gas through said drilling member while continuing the operation of such drilling member in order to increase the drilling rate; and passing an explosive device through such drilling member for generating an explosion in the well bore during the drilling operation.

2. A method for drilling a well bore into earth formations comprising the steps of: operating a drilling member in the earth formation for removing portions from the earth formation; subjecting the area about such drilling member to alternate slugs of liquid and gas in order to enhance removal of portions of the earth formation; and generating explosive energy in the area about such drilling member while continuing the operation of such drilling member.

3. A method for drilling earth formations comprising the steps of: operating a rotating bit within a bore in earth formations to remove portions thereof; passing slugs of a liquid and gas alternately into the bore about such bit for enhancing the removal of such portions to the surface; and detonating an explosive in the bore while op erating the bit in the bore.

4. The method of claim 3 wherein said drilling liquid is comprised of materials having a density greater than water.

5. The method of claim 3 wherein said gas is air.

6. In a rotary drilling operation, an improved method of drilling into earth formations which comprises: rotating drill pipe and a bit thereon to drill a borehole into earth formations; passing a drilling fluid through such drill pipe and bit to the lower end of the borehole and then to the surface on the outside of the pipe to remove formation cuttings to the surface and to maintain back pressure on the formations traversed by such drilling operation; alternating the drilling fluid constituents between a fluid and gas to provide alternate slugs of liquid and gas about the bit in order to increase the drilling rate; and while continuing rotation of the bit in'the borehole, introducing an explosive capsule into the drilling fluid at the surface for movement in the fluid through the drill pipe to the lower end of the borehole where the capsule is detonated upon contact with the lower end of the borehole.

7. The method of claim 6 wherein the explosive is introduced into the liquid slug of the drilling fluid.

8. An apparatus for drilling a well bore in earth formations comprising: rotary earth boring means for removing portions of an earth formation; fluid circulating means in communication with said boring means for passing a drilling fluid into the well bore; automatically operated valve means in said fluid circulating means for introducing a stream comprised of alternate slugs of gas and liquid into such fluid circulation means so that the environment surrounding said earth boring means will substantially alternate between slugs of liquid and gas; and means in said fluid circulating means for introducing an explosive device into said fluid stream.

9. The apparatus of claim 8, and further including a gas and liquid pump means in communication with the fluid circulating means, and accumulator means in said fluid circulating means for accumulating at least one of the gas and liquid slugs before introducing such slug into the well bore.

10. The apparatus of claim 8 wherein said explosive device introducing means is positioned downstream of said valve means in said fluid circulating means.

References Cited UNITED STATES PATENTS 1,867,832 7/1932 Hill -69 2,880,965 4/1959 Bobo 17569 2,951,680 9/1960 Champ 175-69 3,022,729 2/1962 Robinson 1754.5 X 3,083,778 4/1963 Friedman 1754.5 3,190,372 6/1965 Johnson 175-4.5 3,268,017 8/1966 Yarbrough 17569 X 3,387,672 6/1968 Cook 175-69 2,836,246 5/1958 Hoch 175-205 X NILE C. BYERS, 1a., Primary Examiner U.S. Cl. X.R. 175205 

